For a two-cycle engine in which a fuel-air mixture inside a combustion chamber ignites and explodes, pushing down the piston, the exhaust port first opens to begin exhausting the combustion gas, and then the scavenging port opens, introducing the fuel-air mixture supplied to the crankcase into the combustion chamber to exhaust the remaining combustion gas, a known alternative includes an air passage that is connected to the scavenging passage linking the crankcase and the combustion chamber. When the scavenging port opens, the scavenging air in the air passage is first introduced into the combustion chamber to exhaust the combustion gas, and then the fuel-air mixture in the crankcase is introduced into the combustion chamber via the scavenging passage.
The air valve for controlling the scavenging air flow rate, provided in the air passage, and the throttle valve for controlling the output of the carburetor, which is a fuel-air mixture formation means provided in the fuel-air mixture passage connected to the crankcase, must be coordinated with each other in order to prevent incomplete combustion and to stabilize engine operation. To achieve such an objective, the air passage and the fuel-air mixture passage are positioned adjacent to each other vertically, and then the air valve and the throttle valve are integrated to make them work together as described in JP H110-252565; or in configurations in which the air passage and the fuel-air mixture passage are positioned in other ways, the air valve and the throttle valve work together via a linking mechanism as described in JP H9-125966 and JP H9-287521.
In the aforementioned configuration in which the air passage and the fuel-air mixture passage are positioned adjacent to each other vertically and are integrated, the interlocking mechanism for the air valve and the throttle valve is either unnecessary or can be extremely simple. Thus, it is easy to keep these two valves coordinated at proper degrees of opening. However, such a configuration places significant restrictions on the carburetor structure and on the positioning of various mechanisms, significantly reducing the degree of design freedom and making it impossible to incorporate it into an existing carburetor as is, thereby resulting in inconvenience.
On the other hand, the configuration in which the air valve and the throttle valve work together via a linking mechanism can accept either an existing or a freely-designed carburetor. However, manufacturing variations in the linking mechanism and the required clearance in the link junction make it difficult to maintain a proper opening relationship between the air valve and the throttle valve. A particular concern exists in that such a configuration may upset the air/fuel ratio in that partial load region, thereby lowering engine performance.